The anthraquinone process (also called the autoxidation process or the Riedl-Pfleiderer process) for the manufacture of hydrogen peroxide (H.sub.2 O.sub.2) is well known. It is described, for example, in Riedl, U.S. Pat. No. 2,158,525, and in the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed., Volume 13, Wiley, New York, 1981, pp. 15-22. The process is shown schematically below. ##STR1##
In this process, an alkyl-substituted anthraquinone (I), preferably a 2-alkylanthraquinone, known as a working compound, is dissolved in a suitable solvent or solvent mixture to form a working solution. The working solution is catalytically reduced to form the corresponding dihydroanthraquinone (II). The dihydroanthraquinone is separated from the hydrogenation catalyst and exposed to an oxygen-containing gas, usually air, to produce hydrogen peroxide and reform the anthraquinone (I). The hydrogen peroxide is extracted from the solvent with water, purified, and concentrated. The extracted solvent is recycled to the hydrogenation step to reform the dihydroanthraquinone (II) and continue the process.
During the hydrogenation and oxidation steps the working compound can undergo a number of secondary reactions. Reduction of one of the aromatic rings produces a tetrahydro-anthraquinone (III). Although the tetrahydro-anthraquinone is effective in producing hydrogen peroxide and is considered to be part of the working compound, it is the apparent precursor of a tetrahydro-anthraquinone epoxide (IV). The tetrahydro-anthraquinone epoxide is ineffective in producing hydrogen peroxide. It builds up in the working solution as the working solution is recycled through the process. Eventually it must be converted to working compound or removed to maintain the composition of the working solution within prescribed limits. ##STR2##
Numerous processes for regenerating the working solution have been disclosed. For example, Sprauer, U.S. Pat. No. 2,739,875, discloses regenerating the working solution by heating it with either activated alumina or activated magnesia. Logan, U.S. Pat. No. 3,912,766, discloses that a degraded working solution may be regenerated by subjecting the solution to at least two different regeneration agents differing in at least one significant characteristic. The treatments suggested included activated alumina for the hydro (reduced) phase, and aqueous sodium hydroxide for the neutral (oxidized) phase. Shin, U.S. Pat. No. 3,965,251, discloses regenerating a working solution by contacting it at 130.degree. C. or higher with a platinum metal and an olefinic compound whose hydrogenation compound is gaseous at the operating temperature. Ochoa, U.S. Pat. No. 4,566,998, discloses a process for regenerating a mixed oxidized and hydrogenated working solution with an alkaline, activated alumina containing sodium oxide at 145.degree. C.
Although the epoxide can be converted back to working compound, these conversions are not quantitative; usually there is a significant loss of working compound. Because anthraquinone is expensive, this loss is a significant expense in the process. In addition, the epoxide is usually converted to working compound in the hydro phase or reduced phase (after hydrogenation), but dehydrogenation of tetrahydro-anthraquinone to the anthraquinone usually takes place in the neutral, or oxidized, phase (before hydrogenation) so that it necessary to treat both phases of the process. Thus, a need exists for a better method for converting tetrahydro-anthraquinone epoxide to working compound and which, preferably, also converts the tetrahydro-anthraquinone to the corresponding anthraquinone, so that it is only necessary to treat one phase of the process.